Rafting vesicle

Just as organisms move, so too do their cells and even the organelles within them. For example, vesicles — intracellular transport vehicles — move from organelle to organelle to reach the plasma membrane that encloses a cell, or in the reverse direction.

And just as animals need their skeletons for movement, there's a suspicion that the movement of vesicles relies in some way on the network of proteins that make up a cell's intracellular skeleton — the cytoskeleton. Indeed, some pathogens, such as Listeria monocytogenes, take over the cytoskeletal protein actin to propel themselves around the cell. So it seemed plausible that the cell itself makes use of its skeleton for vesicle movement. Now, however, A. L. Rozelle and colleagues offer evidence for a direct linkage between vesicle transport and actin (Current Biology 10, 311–320; 2000).

Rozelle et al. started by looking at a lipid molecule called phosphatidylinositol-4,5-bisphosphate (or PIP₂ for short), which regulates both cytoskeletal and vesicle-trafficking proteins. An enzyme that results in the production of PIP₂ led to the formation of actin-containing ‘comet tails’ in mouse fibroblast cells. In the pictures reproduced here, green staining identifies these comets, and the red circular shapes at the ends of the comets are vesicles.

The authors also found that comet formation relies on the cell recruiting certain effector proteins to the vesicles found at the head of comet tails. So, PIP₂ and these proteins may work together in some way to generate comets — probably, Rozelle et al. suggest, through two well-known actin-polymerizing proteins.

Rozelle et al. next showed that the vesicles at the heads of the comets were derived from an organelle called the Golgi complex or from the plasma membrane. These and other cellular membranes contain microregions, known as ‘rafts’ because cholesterol and sphingolipids ‘float’ in tight-knit groups in these areas. The vesicles at the comet heads mainly budded off from these rafts.

It seems certain that cells do use their cytoskeleton to move vesicles about. But what exactly does actin do? It might help the vesicle-budding process, or it could be a road along which motor proteins carry vesicles. Or, as suggested by Rozelle et al., the comets themselves might push the vesicles along.